Low NOx combustion

ABSTRACT

A furnace has a process chamber and a burner that is operative to fire a flame into the process chamber with the influence of a flame stabilizer. A bypass apparatus is configured to inject an unignited stream of premix into the process chamber without the influence of a flame stabilizer.

TECHNICAL FIELD

This technology relates to a method and apparatus for operating a furnace.

BACKGROUND

As shown partially in the schematic view of FIG. 1, a prior art furnace 10 has a wall structure 12 defining a process chamber 15. The process chamber 15 is sized to contain a load to be heated. A burner 16 fires into the process chamber 15. The burner 16 includes a mixer tube 18 and a burner tile 20. The burner tile 20 defines a reaction zone 21 between the mixer tube 18 and the process chamber 15.

In operation of the burner 16, streams of fuel and oxidant form a combustible mixture known as premix as they flow together through the mixer tube 18 toward and into the reaction zone 21 through the open outer end 22 of the mixer tube 18. An igniter 24 ignites the premix in the reaction zone 21 so that combustion proceeds with a flame that extends across the reaction zone 21 and through a port 25 that communicates with the process chamber 15.

A burner, by definition, includes a flame stabilizer which functions to hold the flame in the desired location by inhibiting flashback and blow off. The premix flows from left to right as viewed in FIG. 1, while the flame propagates in the opposite direction toward the source of premix. Flashback occurs when the flame advances too far into the mixer tube 18. This can be inhibited by controlling the premix speed relative to the flame speed. Blow off occurs when the flame is driven too far from the mixer tube 18. This can be inhibited by a flame stabilizer which, as known in the art, may comprise an obstruction that is placed in the premix flow path to slow the premix and thereby help to ensure that the premix speed does not overly exceed the flame speed. A flame stabilizer can further inhibit blow off by inducing turbulence that includes recirculation toward the stabilizer. The recirculating products of combustion help to anchor the flame by maintaining ignition of the premix near the stabilizer.

In the example shown in FIG. 1, the burner 16 includes a flame stabilizer 30 in the form of a circular metal plate. The plate 30 is preferably located coaxially within the mixer tube 18 at a location spaced a short distance inward from open outer end 22, but could be located farther back inward from the end 22 or a short distance outward from the end 22, as known in the art. The plate 30 extends fully across the inside of the mixer tube 18, as shown in enlarged detail in FIG. 2, but an alternative arrangement could provide an annular flow area radially between the plate 30 and the surrounding tube 18. In either case, small ports 31 extend through the plate 20 to direct jets of premix into the reaction zone 21. The jets of premix together induce recirculation toward the outer side surface 32 of the plate 30, and also define zones of lower premix flow velocity in the spaces between the jets. The plate 30 thus functions to inhibit blow off by anchoring the flame at or near the surface 32. The plate 30 also functions to inhibit flashback upstream of the plate 30, which is to the left as viewed in FIGS. 1 and 2, by enabling the upstream premix speed to exceed the flame speed as needed.

The furnace 10 can be operated in a mode in which diffuse combustion occurs in the process chamber 15 in the absence of a flame in the reaction zone 21. This can occur without the use of the igniter 24 if the premix is injected through the reaction zone 21 and into the process chamber 15 when the process chamber 15 is at or above the autoignition temperature of the premix. The diffuse combustion mode produces less NOx because the furnace gases circulating throughout the volume of the process chamber 15 absorb heat from the burning reactants. This results in a lower flame temperature which, in turn, results in less NOx formation.

Another prior art furnace 40 with a process chamber 43 and a burner 44 is shown partially in the schematic view of FIG. 3. The burner 44 of FIG. 3 raises the process chamber 43 to the auto ignition temperature of the premix, and is then shut off. Diffuse combustion follows as separate streams of fuel and air are injected into the process chamber 43 through fuel and air injectors 46 and 48, respectively. Combustion in the diffuse mode of FIG. 3 produces less NOx than combustion in the diffuse mode of FIG. 1. This is because the reactants of FIG. 3 are injected directly into the process chamber 43, whereas the reactants of FIG. 1 must first move through the reaction zone 21 before reaching the process chamber 15. Direct injection into the process chamber 43 enables the reactants of FIG. 3 to become more extensively diluted by furnace gases as they mix together to form a combustible mixture prior to reaching the autoignition temperature. When the diluted reactants ignite, the greater amount of diluant absorbs more heat to suppress the flame temperature and thereby to suppress the formation of NOx.

SUMMARY

The claimed invention provides an apparatus for use with a furnace process chamber and a burner. The burner, which includes a flame stabilizer, is operative to fire into the process chamber with flame stabilization. The claimed invention comprises a premix injection apparatus configured to inject unignited premix into the process chamber without flame stabilization. In the absence of a stabilized flame at the premix injection apparatus, the furnace can operate with diffuse combustion more uniformly throughout the process chamber with correspondingly less NOx formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of parts of a prior art furnace.

FIG. 2 is an enlarged partial view of parts of the furnace of FIG. 1.

FIG. 3 is a schematic view of parts of another prior art furnace.

FIG. 4 is a schematic view of parts of a furnace configured according to the claimed invention.

FIG. 5 is a schematic view of parts of another furnace configured according to the claimed invention.

FIG. 6 is a schematic view of parts of yet another furnace configured according to the claimed invention.

DETAILED DESCRIPTION

The furnaces illustrated schematically in FIGS. 4, 5 and 6 have parts that are examples of the elements recited in the apparatus claims, and can be operated in steps that are examples of the elements recited in the method claims. The following description thus includes examples of how a person of ordinary skill in the art can make and use the claimed invention. It is presented here to provide enablement and best mode without imposing limitations that are not recited in the claims. The various parts as shown, described, and claimed, may be of either original or retrofitted installation as required to accomplish any particular implementation of the claimed invention.

The furnace 100 of FIG. 4 includes a wall structure 110. The wall structure 110 defines a process chamber 115 that is sized to contain a load to be heated. A burner 116 is mounted on the wall structure 110 and is operative to fire into the process chamber 115. A reactant supply system 120 includes lines and valves that provide the burner 116 with fuel from a fuel source 122, which is preferably a supply of natural gas, and with oxidant from an oxidant source 124, which is preferably an air blower. The reactant supply system 120 also transmits fuel and oxidant from the sources 122 and 124 to a bypass apparatus 126 that delivers those reactants to the process chamber 115 separately from the burner 116. A controller 130 operates the reactant supply system 120 to control combustion in the process chamber 115.

Although a nozzle mix burner could be used, this burner 116 is a premix burner with a mixer tube 140 and a burner tile 142. The burner tile 142 defines a reaction zone 145 between the mixer tube 140 and a port 147 leading to the process chamber 115. The burner 116 also includes a flame stabilizer 148. The flame stabilizer 148 could be configured in any suitable manner known in the art, but is preferably the same as the flame stabilizer 30 described above with reference to FIGS. 1 and 2.

As further shown schematically in FIG. 4, the burner 116 includes a mixer body 160 that is coupled to the reactant supply system 120. The mixer body 160 includes an oxidant coupling 162, a fuel coupling 164, and an internal fuel line 166. The internal fuel line 166 extends from the fuel coupling 164 into the inner end of the mixer tube 140. The oxidant coupling 162 communicates with the inner end of the mixer tube 140 through an oxidant plenum 165 within the mixer body 160.

The reactant supply system 120 has fuel and oxidant supply lines 170 and 172. It also has a plurality of branch lines with flow control valves. A first branch line 174 extends from the fuel supply line 170 to the fuel coupling 164 at the burner 116. A first valve 176 controls the flow of fuel through the first branch line 174. A second branch line 178 extends from the oxidant supply line 172 to the oxidant coupling 162 at the burner 116. A second valve 180 controls the flow of oxidant through the second branch line 178. Third and fourth branch lines 184 and 186 likewise have third and fourth valves 188 and 190. Those branch lines 184 and 186 extend from the fuel and oxidant supply lines 170 and 172 to the bypass apparatus 126.

Unlike the burner 116, the bypass apparatus 126 does not include a flame stabilizer. It is instead configured to inject unignited premix into the process chamber 115 without the influence of a flame stabilizer, whereby the resulting ignition and combustion of the premix proceeds without flame stabilization. In this particular example, the bypass apparatus 126 includes a mixer body 200 and a premix injector tube 204. A mixing chamber 205 is defined within the mixer body 200. The mixing chamber 205 communicates with the third branch line 184 through a fuel coupling 206, and communicates with the fourth branch line 186 through an oxidant coupling 208. The premix injector tube 204 has an open inner end 210 at the mixing chamber 205 and an open outer end 212 at the process chamber 115. There is no structure in the bypass apparatus 126 for slowing the flow of premix at a location between the inner end 210 of the injector tube 204 and the region of the process chamber 115 where the premix emerges from the open outer end 112 of the injector tube 204. Nor is there any structure for inducing upstream recirculation toward or within the injector tube 204. Instead, the bypass apparatus 126 is free of any structure configured for the purpose of inhibiting flashback or blow off of a flame propagating in a direction inwardly of the premix injector tube 204.

The controller 130 has hardware and/or software configured to control combustion in the process chamber 115 by selective use of the burner 116 and the bypass apparatus 126. The controller 130 may thus comprise any suitable programmable logic controller or other control device, or combination of control devices, that is programmed or otherwise configured to perform as recited in the claims. As the controller 130 carries out those instructions it actuates the valves 176, 180, 188 and 190 to initiate, regulate and terminate flows of reactant streams through the reactant supply system 120.

In a startup mode of operation, the controller 130 actuates an igniter 220 in the reaction zone 145, and opens the first and second valves 176 and 180 while maintaining the third and fourth valves 188 and 190 in closed conditions. This transmits streams of fuel and oxidant to the mixer body 160 at the burner 116 while blocking transmission of fuel and oxidant to the process chamber 115 through the bypass apparatus 126. The reactant streams transmitted to the burner 116 form premix as they mix and flow together through the mixer tube 140 toward the reaction zone 145. The premix is ignited upon emerging form the mixer tube 140 to form a flame that projects through the reaction zone 145 and into the process chamber 115 through the port 147 under the influence of the flame stabilizing structure 148. As a safety precaution the flame is monitored by the controller 130 and a flame sensor 222. The flame sensor 222 may comprise any suitable device that is operative to detect the presence of a flame stabilized at a burner. Such devices that are known in the art include, for example, a flame rod, a UV (ultraviolet) flame detector, an IR (infrared) flame detector, a thermopile, and an acoustic flame sensor.

When a temperature sensor 224 indicates that the temperature in the process chamber 115 has risen to a level at or above the autoignition temperature of the premix, the controller 130 responds by shifting from the startup mode of operation to a diffuse combustion mode of operation. The controller 130 shifts to the diffuse combustion mode by closing the first and second valves 176 and 178 to block the transmission of fuel and oxidant to the burner 116, and by opening the third and fourth valves 188 and 190 to transmit fuel and oxidant to the bypass apparatus 126. The fuel and oxidant then mix together in the chamber 205 to form unignited premix that is transmitted through the injector tube 204 for injection into the process chamber 115. Importantly, the premix emerging from the open outer end 212 of the injector tube 204 flows into the process chamber 115 without the influence of a flame stabilizer. This enables autoignition of the premix to result in diffuse combustion more uniformly throughout the process chamber 115. Since the bypass apparatus 126 does not produce a stabilized flame, the furnace 100 does not have a flame sensor in operative association with the bypass apparatus 126, but instead employs the temperature sensor 224 as a safety device to monitor diffuse combustion in the process chamber 115.

The furnace 300 of FIG. 5 also is configured according to the claimed invention. A wall portion 302 of the furnace 300 defines a process chamber 305. A premix burner 306 with an igniter 308 is operative to fire through a reaction zone 315 and into the process chamber 305 under the direction of a controller 320. This is accomplished by actuating a reactant supply system 330 to transmit streams of fuel and oxidant from their sources 332 and 334 to a mixer tube 336 in the burner 306 in the manner described above with reference to the furnace 100 of FIG. 4.

As shown in the drawing, the furnace 300 differs from the furnace 100 by including a bypass apparatus 340 that is structurally combined with the burner 306 rather than structurally separate from the burner 306. Specifically, the bypass apparatus 340 includes a premix injector tube 342 that extends through the burner tile 344. The outer end 346 of the injector tube 342 is open to the process chamber 305. The inner end 348 of the injector tube 342 is open to an oxidant plenum 349 within a bypass mixer body 350 that is joined with the mixer body 352 at the burner 306.

An oxidant branch line 360 with a flow control valve 362 extends from an oxidant supply line 364 to an oxidant coupling 366 at the bypass mixer body 350. A fuel branch line 370 with a flow control valve 372 extends from a fuel supply line 374 to a fuel coupling 376 at the bypass mixer body 350. The flow path of fuel from the source 332 to the injector tube 342 is completed by an internal fuel line 380 that extends from the fuel coupling 376 into the inner end 348 of the injector tube 342.

The controller 320 initiates a startup mode of operation by actuating the igniter 308 and opening the fuel and oxidant flow control valves 390 and 392 that serve the burner 306. The resulting flame is stabilized by a flame stabilizer 394 and monitored by a flame sensor 396 throughout the startup mode. The flow control valves 362 and 372 that serve the bypass apparatus 340 are maintained in closed conditions throughout the startup mode.

When the temperature in the process chamber 305 reaches a level at or above the autoignition temperature of the premix, as indicated by a temperature sensor 398 in the process chamber 305, the controller 320 responds by shifting from the startup mode of operation to a diffuse combustion mode of operation. This is accomplished by closing the valves 390 and 392 that serve the burner 306, and by opening the valves 362 and 372 that serve the burner bypass apparatus 340. Streams of fuel and oxidant then flow into the open inner end 346 of the injector tube 342 and form premix as they flow together along the length of the tube 342 toward the process chamber 305.

As shown in the drawing, there is no flame stabilizer in the premix flow path that extends through the injector tube 342 and into the process chamber 305 from the open outer end 348 of the injector tube 342. There is no flame sensor operatively associated with the bypass apparatus 340. Therefore, in operation of the bypass apparatus 340, an unignited stream of premix flows through the open outer end 348 of the injector tube 342 and into the process chamber 305 without the influence of a flame stabilizer. Diffuse combustion in the process chamber 305 proceeds accordingly, and is monitored for safety by the temperature sensor 398.

The claimed invention further provides other operational modes in addition to the startup and diffuse combustion modes described above. For example, furnace startup is not the only time at which the burner can be on while the bypass apparatus is off. That flame stabilization mode can be continued after startup, and can be alternated with the non-stabilization mode in which the burner is off and the bypass apparatus is on. It may also be appropriate to operate in a mode in which the burner and the bypass apparatus are on simultaneously. Overall target rates of fuel and oxidant injection could then be provided in partial rates at the burner and the bypass apparatus. In this regard, FIG. 6 shows a modification of the furnace 300 in which fuel and oxidant valves are arranged to distribute the overall rates of fuel and oxidant injection between the burner mixer body 352 and the bypass mixer body 350. In this arrangement a directional control valve 400 for fuel is shiftable to initiate, regulate, and terminate flows of fuel to only the burner mixer body 352, to only the bypass mixer body 350, or to both throughout a range of proportional conditions. A directional control valve 402 for oxidant is shiftable in the same manner.

This written description sets forth the best mode of carrying out the invention, and describes the invention to enable a person of ordinary skill in the art to make and use the invention, by presenting examples of the elements recited in the claims. The patentable scope of the invention is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples, which may be available either before or after the application filing date, are intended to be within the scope of the claims if they have structural or method elements that do not differ from the literal language of the claims, or if they have equivalent structural or method elements with insubstantial differences from the literal language of the claims. 

1. An apparatus configured for use with a furnace process chamber and a burner that is operative to fire into the process chamber with flame stabilization, said apparatus comprising: a premix injection apparatus that is configured to inject unignited premix into the process chamber without flame stabilization.
 2. An apparatus as defined in claim 1 wherein the premix injection apparatus includes a mixer chamber configured to form premix as fuel and oxidant streams mix together upon flowing into the mixer chamber, and includes a premix injector tube configured to extend from the mixer chamber to the process chamber to transmit the premix to the process chamber.
 3. An apparatus as defined in claim 1 wherein the premix injection apparatus includes a premix injector tube configured to receive separate fuel and oxidant streams in an inner end of the tube, to form premix as the fuel and oxidant streams mix upon flowing through the tube, and to inject the premix into the process chamber from an open outer end of the tube.
 4. An apparatus as defined in claim 1 further configured for use with a reactant supply system that controls the transmission of fuel and oxidant to the process chamber, and a temperature sensor in the process chamber, said apparatus further comprising a controller configured a) to operate the reactant supply system in a flame stabilization mode in which the reactant supply system transmits fuel and oxidant to the process chamber through the burner but prevents transmission of unignited premix to the process chamber through the premix injection apparatus, b) to operate the reactant supply system in a non-stabilization mode in which the reactant supply system blocks transmission of fuel and oxidant to the process chamber through the burner but transmits unignited premix to the process chamber through the premix injection apparatus, and c) to respond to the temperature sensor by shifting from the flame stabilization mode to the non-stabilization mode at a time when the temperature of the process chamber is not less than the autoignition temperature of the premix.
 5. An apparatus as defined in claim 4 wherein the flame stabilization mode is a startup mode.
 6. An apparatus as defined in claim 1 further configured for use with a reactant supply system that controls the transmission of fuel and oxidant to the process chamber, said apparatus further comprising a controller configured to operate the reactant supply system in a mode in which the reactant supply system transmits fuel and oxidant to the process chamber through the burner and simultaneously transmits unignited premix to the process chamber through the premix injection apparatus.
 7. An apparatus comprising: a furnace structure defining a process chamber; a burner, which includes a flame stabilizer, that is operative to fire a flame into the process chamber; and a bypass apparatus that is configured to inject unignited premix into the process chamber along a flow path that bypasses the burner and is free of a flame stabilizer.
 8. An apparatus as defined in claim 7 wherein the bypass apparatus includes a mixer chamber configured to form premix as fuel and oxidant streams mix together upon flowing into the mixer chamber, and includes a premix injector tube extending from the mixer chamber to the process chamber to transmit the premix to the process chamber.
 9. An apparatus as defined in claim 7 wherein the bypass apparatus includes a premix injector tube configured to receive separate fuel and oxidant streams in an inner end of the tube, to form premix as the fuel and oxidant streams mix upon flowing through the tube, and to inject the premix into the process chamber from an open outer end of the tube.
 10. An apparatus as defined in claim 7 wherein the burner is a premix burner with a mixer tube and a burner tile that defines a reaction zone between the mixer tube and the process chamber, and the flame stabilizer is configured to obstruct the flow of unignited premix from the mixer tube into the reaction zone.
 11. An apparatus as defined in claim 7 further comprising a reactant supply system configured to control the transmission of fuel and oxidant to the process chamber, a temperature sensor in the process chamber, and a controller configured a) to operate the reactant supply system in a burner mode in which the reactant supply system transmits fuel and oxidant to the process chamber through the burner but prevents transmission of unignited premix to the process chamber through the bypass apparatus, b) to operate the reactant supply system in a bypass mode in which the reactant supply system blocks transmission of fuel and oxidant to the process chamber through the burner but transmits unignited premix to the process chamber through the bypass apparatus, and c) to respond to the temperature sensor by shifting from the burner mode to the bypass mode at the time when the temperature of the process chamber is not less than the autoignition temperature of the premix.
 12. An apparatus as defined in claim 11 wherein the burner mode is a startup mode.
 13. An apparatus as defined in claim 7 further comprising a reactant supply system that controls the transmission of fuel and oxidant to the process chamber, and a controller configured to operate the reactant supply system in a mode in which the reactant supply system transmits fuel and oxidant to the process chamber through the burner and simultaneously transmits unignited premix to the process chamber through the bypass apparatus.
 14. A method of operating a furnace having a process chamber, a burner that is operative to fire into the process chamber with flame stabilization, and a reactant supply system configured to control the transmission of fuel and oxidant to the process chamber, said method comprising: forming unignited premix from the fuel and oxidant and injecting the unignited premix into the process chamber to induce autoignition and combustion of the premix in the process chamber without flame stabilization.
 15. A method as defined in claim 14 wherein the unignited premix is injected into the process chamber in a non-stabilization mode of operation in which fuel and oxidant are not transmitted to the process chamber through the burner, and the non-stabilization mode is preceded by a flame stabilization mode in which fuel and oxidant are transmitted to the process chamber through the burner but the unignited premix is not injected into the process chamber, with the intervening step of shifting from the flame stabilization mode to the non-stabilization mode at a time when the temperature of the process chamber is not less than the auto ignition temperature of the premix.
 16. A method as defined in claim 15 wherein the flame stabilization mode is a startup mode.
 17. A method as defined in claim 14 wherein the unignited premix is injected into the process chamber to induce autoignition and combustion of the premix in the process chamber without flame stabilization in an operational mode in which the burner is simultaneously fired into the process chamber with flame stabilization.
 18. A method of operating a furnace having a process chamber, a burner that is operative to fire into the process chamber with the influence of a flame stabilizer, and a reactant supply system configured to control the transmission of fuel and oxidant to the process chamber, said method comprising: forming unignited premix from the fuel and oxidant and injecting the unignited premix into the process chamber along a flow path that bypasses the flame stabilizer to induce autoignition and combustion of the premix in the process chamber without flame stabilization.
 19. A method as defined in claim 18 wherein the premix is injected into the process chamber in a bypass mode of operation in which fuel and oxidant are not transmitted to the process chamber through the burner, and the bypass mode is preceded by a burner mode in which fuel and oxidant are transmitted to the process chamber through the burner but the premix is not injected into the process chamber, with the intervening step of shifting from the burner mode to the bypass mode at a time when the temperature of the process chamber is not less than the autoignition temperature of the premix.
 20. A method as defined in claim 19 wherein the burner mode is a startup mode.
 21. A method as defined in claim 18 wherein the unignited premix is injected into the process chamber to induce autoignition and combustion of the premix in the process chamber without flame stabilization in an operational mode in which the burner is simultaneously fired into the process chamber with flame stabilization.
 22. A method of retrofitting a furnace having a process chamber and a burner that is operative to fire into the process chamber with flame stabilization, said method comprising: installing a premix injection apparatus that is configured to inject unignited premix into the process chamber without flame stabilization.
 23. A method as defined in claim 22 wherein the apparatus further includes a reactant supply system configured to control the transmission of fuel and oxidant to the process chamber, and a temperature sensor in the process chamber, and said method further comprises installing a controller configured a) to operate the reactant supply system in a flame stabilization mode in which the reactant supply system transmits fuel and oxidant to the process chamber through the burner but prevents transmission of unignited premix to the process chamber through the premix injection apparatus, b) to operate the reactant supply system in a non-stabilization mode in which the reactant supply system blocks transmission of fuel and oxidant to the process chamber through the burner but transmits unignited premix to the process chamber through the premix injection apparatus, and c) to respond to the temperature sensor by shifting from the flame stabilization mode to the non-stabilization mode at a time when the temperature of the process chamber is not less than the auto-ignition temperature of the premix.
 24. A method as defined in claim 23 wherein the flame stabilization mode is a startup mode.
 25. A method as defined in claim 22 wherein the apparatus further includes a reactant supply system configured to control the transmission of fuel and oxidant to the process chamber, and said method further comprises installing a controller that is configured to operate the reactant supply system in a mode in which the reactant supply system transmits fuel and oxidant to the process chamber through the burner and simultaneously transmits unignited premix to the process chamber through the premix injection apparatus. 